You need an OHM meter. Then pull the radio out of the dash, test between the radio chassis and a good, metal section of the car....a good spot is the door hinges/bolts/screws in running boards...etc. You should read very close to 0 ohms resistance.
If you don't have a meter, get some wire and attach it to the radio chassis at the back brace, and then connect ( touch) the other end to a good metal section of the interior or dash while the car is running. Test different areas and listen for the noise as you go. Also, move the radio in and out to see if the noise increases or decreases.
It's almost always a ground problem with a head unit and no amp. Very rarely , it can be the head unit itself.
Also, if the car is in a garage, pull it outside. Ignition noise can be a lot worst from an enclosed area.